Thermionic wave generator (twg)

ABSTRACT

Energy conversion systems that may employ control grid electrodes, acceleration grid electrodes, inductive elements, multi-stage anodes, and emissive carbon coatings on the cathode and anode are described. These and other characteristics may allow for advantageous thermal energy to electrical energy conversion.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/510,611, filed Jul. 12, 2019, which is a continuation of U.S. patentapplication Ser. No. 16/219,515, filed Dec. 13, 2018, now U.S. Pat. No.10,388,496 issued on Aug. 20, 2019, which claims the benefit of andpriority to U.S. Provisional Application No. 62/598,582, filed Dec. 14,2017. The above applications are hereby incorporated by reference intheir entireties.

FIELD

The present invention comprises an efficient, portable, scalable, directthermal to electrical delivery system, which can also incorporate energystorage. Specifically, a compact thermionic energy converter, known as aThermionic Wave Generator (TWG), is described. The TWG uses electrodeassemblies and, optionally, external magnetic assistance to form amoving wave of tightly packed electrons, and subsequently converts thatwave into electrical power via an inductive element/collectorsub-assembly. In certain applications, the TWG functions as astandalone, heat source-agnostic electrical generator. It may alsofunction as part of an energy storage system using combustible fluids,as well as a back-end to a thermally regenerative energy storage system.In the latter application, hydrogen may be stored in a metal hydride,and that hydrogen can regenerate via a variety of reversible processesincluding solar concentration, solar thermal water cracking, andhydrolysis.

BACKGROUND

Distributed Generation (DG)—power generation that is located on-site atthe point of use—is an alternative to the centralized power generationmodel. The DG sector is growing at 15% annually and is predicted reach$500 billion global market size by 2025. DG advantages over thecentralized generation model include high efficiency, disasterresilience, and security. Renewables, such as wind and solar, representa large portion of the DG market, and thermal DG technologies such asreciprocating engines and gas & steam turbines are in widespread use.

SUMMARY

Described herein are thermal energy to electrical energy powerconversion systems, referred to herein as Thermionic Wave Generators(TWGs). TWGs can receive heat energy from any of a variety of heatsources and generate a thermally emitted wave of electrons that can becaptured to generate electrical currents and voltages.

In thermionic converters, direct energy conversion from a heat source ismade possible by using the phenomenon of thermionic emission. An emitterfabricated from a refractory material is heated to a high temperatureand spontaneously begins to boil off electrons into the surroundingspace. Continuously supplied thermal energy drives this flow ofelectrons.

The efficiency of the system is expressed as that percentage of thermalenergy converted to 1) electron emissions, 2) transmission of thoseemissions to a collector for usable electrical power, and 3) loss ofthermal energy into the surroundings.

In a first aspect, energy converters, such as thermionic energyconverters and/or thermionic wave generators are described. In oneexample, an energy converter of this aspect comprises a first electrodefor emitting electrons, such as a first electrode that includes anemissive carbon coating over at least a portion of a surface of thefirst electrode; a second electrode adjacent to first electrode; a thirdelectrode adjacent to the second electrode, such as a second electrodethat is positioned between the third electrode and the first electrode;a fourth electrode for collecting electrons emitted from the firstelectrode, such as a fourth electrode that is positioned so the thirdelectrode is positioned between the second electrode and the fourthelectrode; and a housing defining an enclosed evacuated volume. Inembodiments, the first electrode, the second electrode, the thirdelectrode, and the fourth electrode are positioned within the enclosedevacuated volume. Optionally, a system of this aspect may furthercomprise an inductive element adjacent to the third electrode, such asan inductive element positioned between the third electrode and thefourth electrode. The first electrode may advantageously have a lowerpotential than the fourth electrode so that emitted electrons areaccelerated toward the fourth electrode, for example.

The first electrode may advantageously comprise a cathode, such as a hotcathode useful for emitting thermal electrons. In some embodiments, thefirst electrode comprises a material having a work function of fromabout 0.25 eV to about 3 eV, such as from 0.25 eV to 1.0 eV, from 0.25eV to 1.5 eV, from 0.25 eV to 2.0 eV, from 0.25 eV to 2.5 eV, from 0.5eV to 3.0 eV, from 1.0 eV to 1.5 eV, from 1.0 eV to 2.0 eV, from 1.0 eVto 2.5 eV, from 1.0 eV to 3.0 eV, from 1.5 eV to 2.0 eV, from 1.5 eV to2.5 eV, from 1.5 eV to 3.0 eV, from 2.0 eV to 2.5 eV, from 2.0 eV to 3.0eV, or from 2.5 eV to 3.0 eV. Optionally, the first electrode comprisesa refractory metal. Example refractory metals include Ti, V, Cr, Mn, Zr,Nb, Mo, Tc, Ru, Rh, Hf, Ta, W, Re, Os, Ir, and alloys thereof.

Optionally, the first electrode includes a plurality of microtipsextending from a base surface of the first electrode. Microtips may beuseful, in some embodiments, for reducing an effective work function ofthe first electrode and advantageously allow for emission of thermalelectrons from the first electrode at lower temperatures than would beotherwise expected based on a material of the first electrode or basedon materials properties of the first electrode. Optionally, the firstelectrode includes the emissive carbon coating over at least a portionof the microtips. Example microtips may exhibit a first cross-sectionaldimension at a base surface of the first electrode and a smallercross-sectional dimension at a distance from the base surface.Optionally, the plurality of microtips exhibit a height to width ratioof from about 4 to about 10, such as from 4 to 5, from 4 to 6, from 4 to7, from 4 to 8, from 4 to 9, from 4 to 10, from 5 to 6, from 5 to 7,from 5 to 8, from 5 to 9, from 5 to 10, from 6 to 7, from 6 to 8, from 6to 9, from 6 to 10, from 7 to 8, from 7 to 9, from 7 to 10, from 8 to 9,from 8 to 10, or from 9 to 10. Example microtips may exhibitcross-sectional or height dimensions of from about 50 nm to about 100μm, such as from 50 nm to 100 nm, from 50 nm to 500 nm, from 50 nm to 1μm, from 50 nm to 5 μm, from 50 nm to 10 μm, from 50 nm to 50 μm, from50 nm to 100 μm, from 100 nm to 500 nm, from 100 nm to 1 μm, from 100 nmto 5 μm, from 100 nm to 10 μm, from 100 nm to 50 μm, from 100 nm to 100μm, from 500 nm to 1 μm, from 500 nm to 5 μm, from 500 nm to 10 μm, from500 nm to 50 μm, from 500 nm to 100 μm, from 1 μm to 5 μm, from 1 μm to10 μm, from 1 μm to 50 μm, from 1 μm to 100 μm, from 5 μm to 10 μm, from5 μm to 50 μm, from 5 μm to 100 μm, from 10 μm to 50 μm, from 10 μm to100 μm, or from 50 μm to 100 μm. In specific examples, the plurality ofmicrotips may comprise vertically arrayed carbon nanotubes, diamondoids,or alkaline metal oxides.

Optionally, the first electrode includes a low work function surfaceutilizing a crystalline ceramic electride in which electrons clathratedin subnanometer-sized cages act as a conductive medium. For example,12CaO-7Al₂O₃ (sometimes known as C12A7), is not consumed duringoperation, and can take advantage of the electride's capability of lowwork function and of starting at room temperature without the benefit ofa heater. Such materials may advantageously be useful as or in a firstelectrode since it can emit electrons at low temperatures, such asaround 400 K. Accordingly, in some embodiments, the first electrodecomprises a crystalline ceramic electride, such as 12CaO-7Al₂O₃ or anelectride thereof, for example a coating of a crystalline ceramicelectride. Crystalline ceramic electrides may be formed, for example, bycombining suitable precursors (e.g., CaCO₃ and Al₂O₃ in the case ofC12A7), and heating to high temperature, such as from 1600° C. to 1800°C., in a graphite crucible.

Optionally, the first electrode is fabricated using one or moremicrofabrication techniques. Microfabrication may advantageously allowfor precise control over position, thickness, microtip dimensions,spacings, or the like. Optionally, the first electrode is deposited ontoa MoCu alloy substrate. Example thicknesses for the first electrodeinclude from 10 nm to 20 μm, such as from 10 nm to 20 nm, from 10 nm to50 nm, from 10 nm to 100 nm, from 10 nm to 200 nm, from 10 nm to 500 nm,from 10 nm to 1 μm, from 10 nm to 2 μm, from 10 nm to 5 μm, from 10 nmto 10 μm, from 10 nm to 20 μm, from 20 nm to 50 nm, from 20 nm to 100nm, from 20 nm to 200 nm, from 20 nm to 500 nm, from 20 nm to 1 μm, from20 nm to 2 μm, from 20 nm to 5 μm, from 20 nm to 10 μm, from 20 nm to 20μm, from 50 nm to 100 nm, from 50 nm to 200 nm, from 50 nm to 500 nm,from 50 nm to 1 μm, from 50 nm to 2 μm, from 50 nm to 5 μm, from 50 nmto 10 μm, from 50 nm to 20 μm, from 100 nm to 200 nm, from 100 nm to 500nm, from 100 nm to 1 μm, from 100 nm to 2 μm, from 100 nm to 5 μm, from100 nm to 10 μm, from 100 nm to 20 μm, from 200 nm to 500 nm, from 200nm to 1 μm, from 200 nm to 2 μm, from 200 nm to 5 μm, from 200 nm to 10μm, from 200 nm to 20 μm, from 500 nm to 1 μm, from 500 nm to 2 μm, from500 nm to 5 μm, from 500 nm to 10 μm, from 500 nm to 20 μm, from 1 μm to2 μm, from 1 μm to 5 μm, from 1 μm to 10 μm, from 1 μm to 20 μm, from @μm to 5 μm, from 2 μm to 10 μm, from 2 μm to 20 μm, from 5 μm to 10 μm,from 5 μm to 20 μm, or from 10 μm to 20 μm.

Emissive carbon coatings may be useful, in some embodiments, forreducing an effective work function of the first electrode andadvantageously allow for emission of thermal electrons from the firstelectrode at lower temperatures than would be otherwise expected basedon a material of the first electrode or based on materials properties ofthe first electrode. Example emissive carbon coatings comprise a dopedor undoped amorphous carbon coating. Example emissive carbon coatingcomprises a doped or undoped nanodiamond coating. Optionally, anemissive carbon coating may be doped with one or more of hydrogen,nitrogen, or boron. Optionally, an emissive carbon coating is depositedusing a chemical vapor deposition process or a physical vapor depositionprocess. Example emissive carbon coatings may exhibit a thickness offrom about 50 nm to about 100 μm, such as from 50 nm to 100 nm, from 50nm to 500 nm, from 50 nm to 1 μm, from 50 nm to 5 μm, from 50 nm to 10μm, from 50 nm to 50 μm, from 50 nm to 100 μm, from 100 nm to 500 nm,from 100 nm to 1 μm, from 100 nm to 5 μm, from 100 nm to 10 μm, from 100nm to 50 μm, from 100 nm to 100 μm, from 500 nm to 1 μm, from 500 nm to5 μm, from 500 nm to 10 μm, from 500 nm to 50 μm, from 500 nm to 100 μm,from 1 μm to 5 μm, from 1 μm to 10 μm, from 1 μm to 50 μm, from 1 μm to100 μm, from 5 μm to 10 μm, from 5 μm to 50 μm, from 5 μm to 100 μm,from 10 μm to 50 μm, from 10 μm to 100 μm, or from 50 μm to 100 μm.

The first electrode may have any suitable surface area and lateraldimensions. Example surface area for the first electrode may be fromabout 0.001 cm² to about 50 cm², such as from 0.001 cm² to 0.005 cm²,from 0.001 cm² to 0.01 cm², from 0.001 cm² to 0.05 cm², from 0.001 cm²to 0.1 cm², from 0.001 cm² to 0.5 cm², from 0.001 cm² to 1 cm², from0.001 cm² to 5 cm², from 0.001 cm² to 10 cm², from 0.001 cm² to 50 cm²,from 0.005 cm² to 0.01 cm², from 0.005 cm² to 0.05 cm², from 0.005 cm²to 0.1 cm², from 0.005 cm² to 0.5 cm², from 0.005 cm² to 1 cm², from0.005 cm² to 5 cm², from 0.005 cm² to 10 cm², from 0.005 cm² to 50 cm²,from 0.01 cm² to 0.05 cm², from 0.01 cm² to 0.1 cm², from 0.01 cm² to0.5 cm², from 0.01 cm² to 1 cm², from 0.01 cm² to 5 cm², from 0.01 cm²to 10 cm², from 0.01 cm² to 50 cm², from 0.05 cm² to 0.1 cm², from 0.05cm² to 0.5 cm², from 0.05 cm² to 1 cm², from 0.05 cm² to 5 cm², from0.05 cm² to 10 cm², from 0.05 cm² to 50 cm², from 0.1 cm² to 0.5 cm²,from 0.1 cm² to 1 cm², from 0.1 cm² to 5 cm², from 0.1 cm² to 10 cm²,from 0.1 cm² to 50 cm², from 0.5 cm² to 1 cm², from 0.5 cm² to 5 cm²,from 0.5 cm² to 10 cm², from 0.5 cm² to 50 cm², from 1 cm² to 5 cm²,from 1 cm² to 10 cm², from 1 cm² to 50 cm², from 5 cm² to 10 cm², from 5cm² to 50 cm², or from 10 cm² to 50 cm².

Optionally, the second electrode comprises a control grid electrode.Control grid electrodes may advantageously be used to reduce oreliminate space charge effects in the thermal emission of electrons froman underlying electrode, such as a hot cathode. Control grid electrodesmay have a negative potential relative to an underlying electrode, suchas a potential that may repel electrons. Optionally, a potential appliedto the second electrode varies as a function of time. Optionally, thesecond electrode has a lower potential than the first electrode.Optionally, the second electrode comprises is electrically connected tothe first electrode. Optionally, the second electrode comprises a gridelectrode or electrode array having electrode elements arranged in agrid configuration. For example, the second electrode may covers up to30% (e.g., 0% to 30%) of a surface area of the first electrode.Optionally, 70% or more (e.g., 70% to 100%) of the surface area of thefirst electrode is exposed through apertures or spacing regions of thesecond electrode. Optionally, the first electrode emits electrons thatpass through apertures or spacing regions of the second electrode.Optionally, a potential applied to the second electrode modulates acurrent of electrons between the first electrode and the fourthelectrode. Optionally, a potential applied to the second electrodefocuses electrons between the first electrode and the fourth electrode.Optionally, the second electrode is fabricated using one or moremicrofabrication techniques. Optionally, is fabricated by patterninglayers of conductive and insulating materials in a grid arrangement.Optionally, the second electrode is separated from the first electrodeby an inter electrode spacing of from 5 μm to 400 μm.

Optionally, the third electrode comprises an acceleration gridelectrode. Acceleration grid electrodes are useful for acceleratingelectrons emitted by an underlying electrode, such as a hot cathode andthat may be modulated by an underlying electrode, such as a control gridelectrode. In some examples, the third electrode has a potential thatvaries as a function of time. Example potentials include those having asquare wave modulation with a frequency of from about 1 kHz to about 1MHz, such as from 1 kHz to 5 kHz, from 1 kHz to 10 kHz, from 1 kHz to 50kHz, from 1 kHz to 100 kHz, from 1 kHz to 500 kHz, from 1 kHz to 1 MHz,from 5 kHz to 10 kHz, from 5 kHz to 50 kHz, from 5 kHz to 100 kHz, from5 kHz to 500 kHz, from 5 kHz to 1 MHz, from 10 kHz to 50 kHz, from 10kHz to 100 kHz, from 10 kHz to 500 kHz, from 10 kHz to 1 MHz, from 50kHz to 100 kHz, from 50 kHz to 500 kHz, from 50 kHz to 1 MHz, from 100kHz to 500 kHz, from 100 kHz to 1 MHz, or from 500 kHz to 1 MHz. Examplepotentials include those having a sinusoidal modulation with a frequencyof from about 25 Hz to about 400 Hz, such as from 25 Hz to 40 Hz, from25 Hz to 50 Hz, from 25 Hz to 60 Hz, from 25 Hz to 80 Hz, from 25 Hz to100 Hz, from 25 Hz to 120 Hz, from 25 Hz to 150 Hz, from 25 Hz to 160Hz, from 25 Hz to 200 Hz, from 25 Hz to 240 Hz, from 25 Hz to 300 Hz,from 25 Hz to 400 Hz, from 40 Hz to 50 Hz, from 40 Hz to 60 Hz, from 40Hz to 80 Hz, from 40 Hz to 100 Hz, from 40 Hz to 120 Hz, from 40 Hz to150 Hz, from 40 Hz to 160 Hz, from 40 Hz to 200 Hz, from 40 Hz to 240Hz, from 40 Hz to 300 Hz, from 40 Hz to 400 Hz, from 50 Hz to 60 Hz,from 50 Hz to 80 Hz, from 50 Hz to 100 Hz, from 50 Hz to 120 Hz, from 50Hz to 150 Hz, from 50 Hz to 160 Hz, from 50 Hz to 200 Hz, from 50 Hz to240 Hz, from 50 Hz to 300 Hz, from 50 Hz to 400 Hz, from 60 Hz to 80 Hz,from 60 Hz to 100 Hz, from 60 Hz to 120 Hz, from 60 Hz to 150 Hz, from60 Hz to 160 Hz, from 60 Hz to 200 Hz, from 60 Hz to 240 Hz, from 60 Hzto 300 Hz, from 60 Hz to 400 Hz, from 80 Hz to 100 Hz, from 80 Hz to 120Hz, from 80 Hz to 150 Hz, from 80 Hz to 160 Hz, from 80 Hz to 200 Hz,from 80 Hz to 240 Hz, from 80 Hz to 300 Hz, from 80 Hz to 400 Hz, from100 Hz to 120 Hz, from 100 Hz to 150 Hz, from 100 Hz to 160 Hz, from 100Hz to 200 Hz, from 100 Hz to 240 Hz, from 100 Hz to 300 Hz, from 100 Hzto 400 Hz, from 120 Hz to 150 Hz, from 120 Hz to 160 Hz, from 120 Hz to200 Hz, from 120 Hz to 240 Hz, from 120 Hz to 300 Hz, from 120 Hz to 400Hz, from 150 Hz to 160 Hz, from 150 Hz to 200 Hz, from 150 Hz to 240 Hz,from 150 Hz to 300 Hz, from 150 Hz to 400 Hz, from 160 Hz to 200 Hz,from 160 Hz to 240 Hz, from 160 Hz to 300 Hz, from 160 Hz to 400 Hz,from 200 Hz to 240 Hz, from 200 Hz to 300 Hz, from 200 Hz to 400 Hz,from 240 Hz to 300 Hz, from 240 Hz to 400 Hz, from 300 Hz to 400 Hz,about 25 Hz, about 50 Hz, about 60 Hz, about 80 Hz, about 100 Hz, about120 Hz, about 150 Hz, about 160 Hz, about 200 Hz, about 240 Hz, about300 Hz, or about 400 Hz. Example potentials include those having asquare wave modulation with a first frequency and a second sinusoidalmodulation with a second frequency. Optionally, the third electrodecovers up to 30% of a surface area of the first electrode. Optionally,up to 70% or more of the surface area of the first electrode is exposedthrough apertures or spacing regions of the third electrode. Optionally,the third electrode has a higher potential than the first electrode.Optionally, the first electrode emits electrons that pass throughapertures of the third electrode. Optionally, a potential applied to thethird electrode modulates a velocity of electrons passing throughapertures of the third electrode. Optionally, a potential applied to thethird electrode focuses electrons passing through apertures of the thirdelectrode. Optionally, the third electrode is fabricated using one ormore microfabrication techniques.

As noted above, systems of this aspect may optionally further comprisean inductive element adjacent to the third electrode, such as aninductive element positioned between the third electrode and the fourthelectrode. Example inductive elements optionally comprises an air coreinductor, such as an air core solenoid. Example inductive elementsoptionally comprise a solid core inductor, for example including aferrite core or an iron core. Optionally, an inductive element comprisesa toroidal core inductor or a pot core inductor. Optionally, the firstelectrode emits electrons that pass through a central opening of theinductive element. Electrons passing through a core of the inductiveelement may advantageously induce a current in the inductive elementand/or induce a voltage across the inductive element. Optionally,electrons passing through a core of the inductive element aredecelerated by interactions with the inductive element. Exampleinductive elements comprise a conductive material arranged in a coilshaped geometry, a planar spiral geometry, a zig-zag geometry, or anycombination of these.

The fourth electrode may advantageously function as a cathode orcollector. Optionally, the fourth electrode has a non-planar geometry.Example materials for the fourth electrode include those having a workfunction of from about 0.25 eV to about 3.0 eV, such as from 0.25 eV to1.0 eV, from 0.25 eV to 1.5 eV, from 0.25 eV to 2.0 eV, from 0.25 eV to2.5 eV, from 0.5 eV to 3.0 eV, from 1.0 eV to 1.5 eV, from 1.0 eV to 2.0eV, from 1.0 eV to 2.5 eV, from 1.0 eV to 3.0 eV, from 1.5 eV to 2.0 eV,from 1.5 eV to 2.5 eV, from 1.5 eV to 3.0 eV, from 2.0 eV to 2.5 eV,from 2.0 eV to 3.0 eV, or from 2.5 eV to 3.0 eV. Optionally, the fourthelectrode comprises a plurality of stages. Using multiple stages may beadvantageous to reduce or eliminate secondary electron generation, forexample. Optionally, the fourth electrode comprises an array ofcollector elements. For example, the collector elements in the array mayoptionally be individually positioned above openings within the secondelectrode and the third electrode. The fourth electrode may optionallyhave a second emissive carbon coating, such as an emissive carboncoating comprising a doped or undoped carbon coating. For example, thesecond emissive carbon coating may comprise a doped or undopednanodiamond coating. Optionally, the fourth electrode comprises one ormore recessed regions or one or more raised regions.

The enclosed evacuated volume in which the electrodes may be arrangedmay have any suitable pressure. During operation, low pressure isdesirable, such as a pressure in which the mean free path for electroncollision with gas in the enclosed evacuated volume is less than adistance between the first electrode and the fourth electrode. Inexamples, the enclosed evacuated volume has a pressure of 10⁻⁵ Ton to10⁻¹² Torr, such as from 10⁻⁷ Torr to 10⁻¹² Torr, from 10⁻⁸ Torr to10⁻¹² Torr, from 10⁻⁹ Torr to 10⁻¹² Torr, from 10⁻¹⁰ Torr to 10⁻¹² Torr,from 10⁻¹¹ Torr to 10⁻¹² Torr, from 10⁻⁷ Torr to 10⁻¹¹ Torr, from 10⁻⁸Torr to 10⁻¹¹ Torr, from 10⁻⁹ Torr to 10⁻¹¹ Torr, from 10⁻¹⁰ Torr to10⁻¹¹ Torr, from 10⁻⁷ Torr to 10⁻¹⁰ Torr, from 10⁻⁸ Torr to 10⁻¹⁰ Torr,from 10⁻⁹ Torr to 10⁻¹⁰ Torr, from 10⁻⁷ Torr to 10⁻⁹ Torr, from 10⁻⁸Torr to 10⁻⁹ Torr, or from 10⁻⁷ Torr to 10⁻⁸ Torn Optionally, a getteris positioned within the enclosed evacuated volume for removing gas fromwithin the evacuated volume. Optionally, the housing is hermeticallysealed. Optionally, energy converters of this aspect may furthercomprise a magnetic field source positioned to direct a magnetic fieldbetween the cathode and the anode. Example magnetic field sourcesinclude a permanent magnet or an electromagnet. Optionally, an energyconverter of this aspect may further comprise a magnetic core positionedfor concentrating and guiding the magnetic field between the firstelectrode and the second electrode. For example, magnetic field linesgenerated by the magnetic field source may be arranged along a directionbetween the first electrode and the fourth electrode. Optionallymagnetic lines generated by the magnetic field source may be arrangedalong a direction parallel to an average electron trajectory between thefirst electrode and the fourth electrode.

Energy converters of this aspect may optionally further compriseadditional elements. For example, energy converters of this aspect mayoptionally further comprise a thermal conductor in thermal communicationwith the first electrode, such as thermal conductor for receivingradiant energy or thermal energy to heat the first electrode. Energyconverters of this aspect may optionally further comprise one or moreelectrically insulating spacer elements between adjacent electrodes.Energy converters of this aspect may advantageously optionally furthercomprise a switching power supply for generating voltages applied to oneor more electrodes, such as a switching power supply is in electricalcommunication with the one or more electrodes. Optionally, energyconverters of this aspect may further comprise a control circuit inelectrical communication with one or more electrodes for modulatingpotentials applied to one or more electrodes. Optionally, energyconverters of this aspect may further comprise a dynode positionedadjacent to the third electrode, such as the dynode is positionedbetween the third electrode and the inductive element. Optionally, aninter-element spacing between the dynode and the third electrode is fromabout 0.5 μm to about 200 μm. Optionally, the dynode has a higherpotential than the third electrode. Optionally, the dynode comprises oneor more of a metal foil, synthetic diamond, or a secondary electronemission coating. Energy converters of this aspect may optionallyfurther comprise one or more conductive alloy electrodes for fixingpositions of the first electrode, the second electrode, or the thirdelectrode.

In another aspect, systems are described, such as energy conversionsystems. An example system of this aspect comprises a heat source; and aplurality of energy converters arranged in a spaced configuration aboutthe heat source. Optionally, the plurality of energy converters arearranged radially about the heat source. Optionally, the heat sourcecomprises a combustor. Optionally, the plurality of energy convertersmake up a majority of one or more walls of the combustor. Optionally,multiple energy converters are arranged in a tandem configuration toprovide cascading thermal conversion. Optionally, at least a portion ofone or more walls of the combustor include 3-dimensional surfacefeatures that reflect electromagnetic radiation having wavelengths offrom 0.8 μm to 1.5 μm. Optionally, at least a portion of one or morewalls of the combustor are coated with or comprise aluminum zinc-dopedoxide to suppress emission of black body radiation at wavelengthsgreater than 1.5 μm or from 1.5 μm to 5 μm. Optionally, the combustorhas a temperature of from about 400° C. to about 1100° C. and generatesblack body electromagnetic radiation. Optionally, at least a portion ofthe black body electromagnetic radiation is transmitted and absorbed bythe plurality of energy converters to heat portions of the plurality ofenergy converters. Example energy converters useful with the systems ofthis aspect may comprise any of the energy converters described herein.

Without wishing to be bound by any particular theory, there can bediscussion herein of beliefs or understandings of underlying principlesrelating to the invention. It is recognized that regardless of theultimate correctness of any mechanistic explanation or hypothesis, anembodiment of the invention can nonetheless be operative and useful.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides a schematic illustration of various components andsubassemblies used to fabricate a Thermionic Wave Generator.

FIG. 2 provides a schematic illustration showing a single TWG deviceused for direct energy conversion. The figure is cut to reveal variousinternal components.

FIG. 3 provides a schematic illustration of a TWG device in closerdetail, showing relative locations of various components.

FIG. 4 provides a schematic illustration of a 3D view of control gridand acceleration grid electrodes.

FIG. 5 provides a cross-sectional view of the control grid andacceleration grid electrodes.

FIG. 6A and FIG. 6B provide a schematic illustrations of a film used fora smooth cathode surface and an etched cathode surface.

FIG. 7A and FIG. 7B provide schematic illustrations of an embodiment ofa TWG integrated within a portable power generator.

FIG. 8 provides a circuit diagram for a TWG for AC and DC power outputmodes.

FIG. 9 provides a plot showing current density for a TWG with both anideal grid (grid losses absent) and a grid with loss factors included.

FIG. 10 provides a plot showing conversion efficiency for a TWG withvarying acceleration grid distances.

FIG. 11 provides a plot showing simulated voltage output from a TWGdevice extracting power from an inductive element over time in responseto a current.

FIG. 12 provides a plot showing simulated voltage output from a TWGdevice over a longer period of time.

FIG. 13 provides a plot showing simulated voltage output from a TWGdevice with 60 Hz modulation on the duty cycle of the square wave inputinto the acceleration grid.

FIG. 14 provides a plot of the spectral radiance from the core of acombustor.

DETAILED DESCRIPTION

Described herein are thermal energy to electrical energy conversionsystems, referred to herein as Thermionic Wave Generators (TWGs). TWGscan receive heat energy from any of a variety of heat sources andgenerate a thermally emitted wave of electrons that can be captured togenerate electrical currents and voltages. In their simplestdescription, TWGs include features comparable to a vacuum tube orthermionic converter, but the TWGs described herein include additionaladvantageous characteristics that allow them to function efficiently forthermal conversion. For example, the disclosed TWGs may employ, controlgrid electrodes, acceleration grid electrodes, inductive elements,multi-stage anodes, and emissive carbon coatings on the cathode andanode to allow for advantageous thermal energy to electrical energyconversion.

Overview of Thermionic Converters

In thermionic converters, direct energy conversion from a heat source ismade possible by using the phenomenon of thermionic emission. An emitterfabricated from a refractory material is heated to a high temperatureand spontaneously begins to boil off electrons into the surroundingspace. Continuously supplied thermal energy drives this flow ofelectrons. These electrons must have enough energy to escape the surfacework function of the emitter. Lower surface work functions result inreduced operating temperatures, higher efficiencies, and larger powerdensities.

Electrons have negative charges and, therefore, repel one another. Theemission of electrons into the inter-electrode space creates anegatively charged electric field that repulses neighboring electrons.This results in a choking effect that reduces the flow rate of theemission, known as a ‘space charge’ effect. The collector of the devicecan also emit electrons that travel to the emitter, in some instancesknown as ‘secondary emission’. The effect is a reversal in current thatreduces overall the electrical output. The output voltage of athermionic energy converter is limited to the work functions of thematerials selected for the emitter and the collector. The output voltagein most thermionic generators is a function of the plate or collector(i.e., positive electrode) voltage. When a pickup is present,particularly an inductive pickup, output voltages may become a complexfunction of the pickup's inductance, the beam current value, and thebeam focus. An accelerator grid or anode gun can also play a role.

The viability of a commercial application may depend on the efficienciesof the system. The efficiency of the system is expressed as thatpercentage of thermal energy converted to 1) electron emissions, 2)transmission of those emissions to a collector for usable electricalpower, and 3) loss of thermal energy into the surroundings. The amountof thermal energy emission itself depends chiefly on three factors: thetemperature of the emissive cathode, its work function, and its physicalsurface properties. Emission of any sort takes place as a thresholdevent and increases as temperature increases, as well as when workfunction lowers. In the absence of other interventions, emissions occurin a statistically random manner, with electrons traveling in directionsthat impede efficiency.

Research has been done to improve the work functions of the emitters andcollectors used in thermionic converters, as well as creating schemes toimprove the power densities by reducing the space charge effects betweenthe cathode and anode. For example, a collector/anode made from similarmaterials may be held at a cooler temperature nearby the emitter, andthe emitted electrons begin to condense on the surface of the collector.These electrons supply electrical power by flowing through an electricalload that connects both the emitter and the collector.

Fine screens have also been employed to create a positively charged gateabove the collector that works to neutralize the space charge caused bythe emitted electrons. These so called ‘gated’ thermionic convertersstill suffer from losses due to secondary emissions at the anode,electron emissions traveling into the gate structure, as well as thermallosses, and may be limited to DC power outputs at voltages below 1.0 Vfor each cathode and anode pair.

Control Over Electron Emission

The presently described systems and techniques controls emissions usingkinetic, electrostatic, and magnetic effects. A positively chargedelectrode, which serves as an acceleration grid, attracts the negativelycharged electrons. A secondary negatively charged electrode controls theflow of these electrons. Together these positive and negative electrodesresult in a tightly packed wave consisting of a volume of negativelycharged electrons that moves at accelerated speed from the emissionsite. This rising wave of charge creates a magnetic field in its wake,as well as interacts with its surroundings by way of electrostaticpotentials. Coupled with an inductive element, whose geometry exploitsthese local disturbances of the magnetic field, these electromagneticand electrostatic phenomena can convert the moving electrons' kineticenergy into voltage and/or current.

When the electron beam interacts with the inductor, a magnetic field, aswell as a voltage, are generated in the inductive coil which immediatelyresults in current flow as the magnetic field in the coil collapses. Theinductive element results in an additional voltage between the cathodeand anode. The anode/collector/positive electrode captures the excessenergy of the electrons.

To address power density and efficiency issues, the disclosed system andmethods may systemically utilize five tightly-integrated structures tocreate a moving volume of electrons that travels in waveform from thecathode to the anode of the device. This tight packet of electronsconstitutes a ‘wave’ due to its observed output that is characterized asa sudden surge and then fall of the local charge concentration. Thesefive structures include a low work function emitter (cathode), anon-planar low work function collector/anode, a control electrode, anacceleration electrode, and a final element to convert the energy of theelectron wave into a desired output voltage. Optionally, an inductiveelement or an electrostatic pickup (or both) may be used to extractenergy from the electrons. Example inductive elements include inductorsand example electrostatic pickups include multi-stage collectors, suchas multi-stage depressed collectors.

An inductive element advantageously address two important issues: lowvoltage outputs and secondary emissions. An inductive structurecapturing the wave of electrons permits direct manipulation of theoutput voltage, and results in a higher voltage, mitigation of secondaryemissions, and hence more converted power, than the contact potential ofthe cathode and anode alone. Further, this inductive interaction resultsin a deceleration of the electron wave and thus reduces the possibilityof reflective secondary emissions when the wave encounters the anode. Inaddition, the calculated and software-simulated, non-planar geometry ofthe anode increases the recapture probability of secondary emissions,when present.

Use of the inductive element may employ a cycle of wave-pulses, achievedby modulating the voltage in the acceleration and control gridelectrodes. Pulsed modulation techniques of the duty cycle of thissignal can effectively produce any AC frequency of choice, including analternating output voltage of 60 Hz, making the TWG an ideal backendconverter for close integration with existing power applications. Waferfabrications techniques permit the TWG to achieve both the structuresand the precise, micron-scale features and placements of thesestructures.

Thermionic Wave Generators

Referring to FIG. 1, a schematic illustration of the sub-assemblies ofan example thermionic wave generator (TWG) 100 is shown, indicating howsome embodiments of the fabrication can occur. Cost-Efficientfabrication may be achieved by using wafer fabrication techniques tobuild the complex and high precision components of the TWG 100, forexample. Other embodiments can include electrode elements that are evenfurther integrated with either the cathode, the anode, or the packagingitself. Advantageously, the core components of TWG 100 are placed insideof an evacuated and hermetically sealed package with electricalterminals connecting the internal electrodes to the exterior of thedevice.

The TWG 100 may include a cathode 105, a control grid assembly 110, anda pickup electrode and anode 115. A packaging may enclose variouscomponents of TWG 100 and be assembled as multiple pieces with a sealingarrangement to separate the internal environment from the externalenvironment. Packaging may include first packaging 120 (e.g., arrangedfor housing the cathode 105) and second packaging 125 (e.g., arrangedfor housing the pickup electrode and anode 115). First packaging andsecond packaging may come together to seal the TWG 100. One or moreelectrical terminals may 130 be positioned outside of the housing toprovide for electrical conduction to external components, with afeedthrough configuration (not shown in FIG. 1) provided for makingelectrical connections between the one or more electrical terminals 130and the cathode 105, the control grid assembly 110, the pickup electrodeand anode 115, and/or other components within the packaging.

Referring to FIG. 2 and FIG. 3, which provides schematic illustrationsof an example TWG 200 in a cutaway and a cross-sectional view, the TWG200 can be broken down into three major sub-systems. The TWG's packaging210 seals the internal elements of the TWG from the surroundings, whilealso creating the structure needed to mount electrical conductors,electrical insulators, and permanent magnets, for example. The inside ofthe TWG is evacuated to create a high vacuum of below or about 10⁻⁵Torr, such as from 10⁻⁷ Torr to 10⁻¹² Torr, or generally below 10⁻¹¹Torr. A cathode 220 is mounted on the ‘bottom’ of the TWG 200 and ispositioned to receive radiant thermal energy, such as from a combustionchamber. As a result, the cathode 220 may reach temperatures from about500° C. to about 1100° C.

A system of electrodes 230 comprises a control grid, acceleration grid,inductive element, and anode. These parts work with the cathode 220 toneutralize the space charge effects of the emitted electrons and focusthe electron wave into a width that can pass through an inductiveelement, without making direct contact. The control grid functions toreduce the loss of emitted electrons into the acceleration grid and tocompact the local electron cloud into a narrower column forms an elementof the electron wave. FIG. 4 and FIG. 5 depicts schematic perspectiveand cross-sectional illustrations of the cathode 220 and control gridassembly 260, showing the arrangement of the separate electrodecomponents. Optionally, cathode 220 may be supported by a cathodesubstrate 223. Control grid assembly 260 is illustrated as includingopenings for passing the electron wave emitted by the cathode 220.Control grid assembly 260 is illustrated in FIG. 5 as including acontrol grid 263 (also referred to herein as a control grid electrode)and an acceleration grid 266 (also referred to herein as an accelerationgrid electrode). The direction of magnetic field lines generated bymagnet 250 are indicated by element 255. Radiant thermal energy may bereceived by a bottom side of TWG 200 and transferred to cathode 220, asindicated by elements 225. Average trajectories of electrons thatcomprise the wave are indicated by elements 245.

The cathode 220, also known as an emitter, may comprise, consist of, orconsist essentially of an etched surface that features a dense array ofmicro-tips and edges with aspect ratios (e.g., height to edge width)greater than 4:1. A schematic view of a cathode 220 without micro-tipsis illustrated in FIG. 6A and a schematic view of a cathode 220 withmicro-tips 221 is illustrated in FIG. 6B. These tips 221 and edges focusand improve the electron emission by increasing the local electric fieldintensity. This local electric field reduces the energy barrier of theemitter and permits emission at energies below those required toovercome the work function of the material of cathode 220. The cathode220 is typically made from a MoCu wafer but can use other refractoryalloys in other embodiments. The surface of cathode 220 is coated withan emissive carbon composite coating 222, which may be amorphous. Thiscomposite coating 222 may comprise a nano-diamond film that is depositedwith chemical vapor deposition (CVD) to produce a mostlynon-crystalline, low work function surface that supports large chargeconcentration. In other fabrication methods, especially at smallerscales, atomic layer deposition (ALD) takes the place of CVD. The highvapor phase transition temperature 1of carbon slows the degradation ofthe emitter's surface properties, resulting in a reliablehigh-temperature electron emitter. Various dopants including hydrogen,nitrogen, and/or boron may be used within the coating to enhance theelectron flow within the carbon matrix. The collector or anode 240 ofthe TWG 200 may also coated with this or another carbon compositecoating to reduce its surface work function. A vacuum gap 299 separatesthe cathode 220 from the anode 240.

A control grid assembly 260 shapes electron emissions from the cathode220. Embodiments of control grid assembly 260 may optionally includeboth a negatively charged control grid 263 and a positively chargedacceleration grid 266. The electric potential created by accelerationgrid 266 may accelerate low-energy electrons near the cathode 200 whilealso reducing the energy barrier for additional electrons within thecathode 220. A negatively charged control grid 263 placed directlybetween the acceleration grid 266 and the cathode 220 may deflectelectrons away from the positively charged acceleration grid 266 in sucha way as to mitigate leakage currents. Voltages on the acceleration grid266 may be time-varying, such as from 0 V to 30 V, and the control grid263 may also have a time-varying potential, such as varying from 0 V to−1 V, with voltages optionally referenced to the potential of cathode220. Such voltages may advantageously result in grid leakages below 10%.As illustrated with respect to FIG. 9, the current density of the TWG200 may be closely related to the space charge limited current asexpressed with the Child-Langmuir equation. To account for accelerationgrid's spacing from the cathode as well as leakage/loss, current density[Amps/cm²] may be modeled as

${J_{TWG} = {\left( {1 - r} \right){2.3}35 \times 10^{- 6}\frac{\left( {{\mu V_{g}} + V} \right)^{\frac{3}{2}}}{x_{g}^{2}}}},$

where V is the contact potential between the cathode 220 and anode 240,V_(g) is the acceleration grid voltage, r is a reflectance factor causedby secondary emissions, and x_(g) is the distance the acceleration grid263 is from the cathode 220. Maximum current may be determined, forexample, by Richardson's law, J_(RD)=AT²e^(−ø/kT).

In FIG. 9, the plot shows example current density for a TWG with both anideal (loss free) grid and a grid with loss factors included. The plotdemonstrates that space charge neutralization occurs within the TWG, andthat maximum current densities are limited to the cathode's material andsurface properties.

FIG. 10 provides a plot of example conversion efficiency for a TWG withvarying acceleration grid distances. Grid current leakage is assumed tobe zero for all of the displayed plots. The plots illustrate theadvantage of smaller grid distances in achieving high conversionefficiencies.

In some embodiments, the voltage of the acceleration grid 266 may bemodulated using a square wave signal with a frequency of from about 1 to10 kHz. While the signal of the square wave is ‘low’, the accelerationgrid voltage is zero, and the control grid 263 prohibits thetransmission of the electron wave. When the signal of the wave is‘high’, the positive acceleration grid 266 is activated and generates aninstantaneous acceleration of the emitted electrons. This action formsthe electron wave with electrical current I_(w) and subsequently sendsthe moving wave at speeds fast enough to create the appearance of aninstantaneous current that follows the same frequency, phase, and dutycycle of the square wave, as illustrated with respect to FIG. 11, FIG.12, and FIG. 13. In embodiments, this frequency-driven electronemission, referred to herein as an electron wave, interacts primarilymagnetically with the inductive element 280 to produce a voltage acrossa resistive load, R_(load), or between the anode 240 and the cathode220.

For example, FIG. 11 depicts plot of simulated voltage output (blackcurve, right y-axis) from a TWG device extracting power from theinductive electrode over time. In certain embodiments, the accelerationgrid is switched on and off with a frequency of 854 Hz, resulting in thesquare waveform of the TWG's electron wave current, I_(ω) (grey curve,left y-axis). The voltage V_(out) is the response from the inductiveelectrode interacting to the current I_(ω). The inductive element'sresponse to current I_(ω) can be modified by changing the accelerationgrid's frequency, the inductance of the electrode, the capacitance ofthe electrode, and the resistance of the applied load. The wave may alsobe slowed by the interaction with the inductive electrode, and mayadvantageously result in a reduction in secondary emissions from theanode. As a consequence, the TWG may converge towards an overallconversion efficiency that is not limited to the Carnot efficiency.

FIG. 12 shows an example plot of simulated voltage (black curve, righty-axis) from a TWG device over a large time scale, with electron wavecurrent also shown (grey curve, left y-axis). In this instance theoutput from the inductive coil converges to an apparent DC output with1.0 volts. 1.0 volts may be higher or significantly higher than thecontact potential achieved by conventional thermionic conversiondevices.

The signal input for the acceleration grid 266 is not limited to squarewaves, and other embodiments may use sine or triangular waveforms forexample. In embodiments, multiplexing techniques or pulse parametermodulation is optionally applied to the input signal. In certainembodiments, a modulation can be a 60 Hz sine wave that varies the dutycycle of a square wave. This additional modulation may produce, forexample an alternating output voltage at the inductive element 280 at afrequency of 60 Hz, for example, as illustrated in FIG. 13. FIG. 13shows an example plot of simulated voltage output (black curve, righty-axis) as in FIG. 12, but with 60 Hz modulation on the duty cycle ofthe acceleration grid applied, with corresponding electron wave currentalso shown (grey curve, left y-axis). The resulting voltage outputexhibits a 60 Hz sine wave character and illustrates how theacceleration grid electrode can optionally be operated in unique way todirectly produce AC power without an external power inverter. In variousembodiments, the space immediately above the control grid assembly 260is optionally occupied with an additional shaping electrode that mayfurther accelerate and focus the width of the electron emission. Thiselectrode may achieve this through the use of electrostatic forces ormagnetic interactions.

A magnetic field created within the inter-electrode gap may serve tokeep electrons confined to trajectories that travel in a path from thecathode 220 to the anode 240. A permanent magnet 250 may optionally bepositioned above the electrode assembly and supply a magnetic field,with lines running along directions between the cathode 220 and anode240, as illustrated by element 255 in FIG. 5, that forces electrons toorbit around the field lines to produce a tight spiral. The magneticfield may also advantageously reduce instances of grid leakage. Examplemagnetic fields may have a field strength of from about 0.02 Tesla toabout 0.5 Tesla.

The inductive element 280 may comprise two insulated coupled coils ofconductive wire wound upon a hollow bobbin, the coil being enclosed by acylinder of magnetic material (e.g., a magnetic core) or otherarchitecturally designed magnetic material container that serves tocontain flux lines and improve the coupling of the two coils, forexample. The first coil may extract energy from the electron wave whilethe second coil communicates with an external circuit and provides for atransformation ratio that supports voltage step-down and optimalamperage. The second coil optionally remains separated from the firstcoil and is optionally closer to the collector or anode 240 than thefirst coil. These coils form an induction element 280 that functions asa transformer or coupled inductors capable of producing voltages thatare proportional to the instantaneous change in the electrical currentof the electron wave. To maintain this voltage, the inductive element280 may interact with a frequency-driven current, achieved by modulatingthe acceleration grid voltage with a high-frequency square wave pulsetrain from a DC power supply, as described above. FIGS. 11, 12, and 13illustrate this interaction between electron wave current I_(w) and thevoltage output from the inductive element 280.

Secondary emissions, when present, result in electrons traveling back tothe cathode 220. The non-planar shape of the anode 240 may reducereflectance and secondary emissions from the anode 240. The large areaof the anode 240 may enhance the cooling properties of the anode 240, aswell. In another embodiment, the anode 240 comprises four non-planarstages, such as with progressively lower negative operating voltages. Ifthese multiple stages are present, this element is designated as amultistage depressed collector. Each stage may exert a braking effect onthe electron beam and may extract any residual motive energy notsurrendered to the inductive element. Reducing a surface work functionof the anode 240 with a carbon composite coating, as described above,may further reduce the chance of secondary emissions. Optionally, thismay be used in conjunction with a multistage depressed collector. Amultistage depressed collector bears negative charges, for example. Aseparate ground return circuit returns the electrons in the wave to theemitter. Optionally, the anode 240 may be fabricated as an array ofanode sub-elements, with each sub-element individually positioneddirectly above respective openings in the control grid assembly 260.

The entire TWG 200 may be assembled using an atmospheric brazing processand other hermetic sealing processes. One or more ceramic spacingelements, such as ceramic spacing element 290, may electrically isolatethe cathode 220, components of control grid assembly 280, and anode 240.Cathode 220 and supporting elements, such as thermally conductive and orelectrically insulating elements (e.g., substrate 223) may beconstructed as a cathode assembly 224. Electrical connections may bemade using separate conductive materials. Electrical terminals provideconductive paths for the electrodes of TWG 200. Sealing occurs afterevacuating all gases from the TWG 200. A film of reactive metal 295 mayfunction as a ‘getter’ and maintains the vacuum environment inside theTWG 200 during its lifetime.

FIG. 8 provides a schematic circuit diagram of a TWG, useful forconversion for both direct current (DC) and alternating current (AC)power output modes. A power supply 805 is used to provide a voltage forthe acceleration grid 810 and optionally the control grid 815 relativeto the cathode 825. A small amount of energy may be expended in theconversion process by power supply 805. A load 820 represents the outputvoltage that may be created during the conversion process between thecathode 825 and the cathode 830 by passage of the electron wave 835through the inductive element 840.

Referring next to FIG. 7A and FIG. 7B, a portable power generator 700including a plurality of TWGs 200 is illustrated, including across-sectional view (FIG. 7A), and perspective view (FIG. 7B). Portablepower generator includes a combustor 715, interfacing with TWGs 200 insuch a way as to reduce thermal losses to the surroundings. The centerof the combustor 715, the ‘core’ 717, is where the combustion processtakes place, and radiant energy travels outwards from the core 717,typically in a radial direction from the center of combustor 715. Fuelis supplied from a storage tank 722. Mixing of fuel and air from inlet718 occurs within a nozzle assembly 716, and the mixture is ignitedinside the core 717. Fuel refilling occurs through a gas port (FIG. 7,ITEM 24). The core 717 is a porous structure, comprising refractorymaterials placed in the center of the combustor 715. The porousstructure evenly distributes the thermal energy of combustion processeswhile simultaneously burning all possible fuel molecules, for example.During the assembly of the combustor 715, air is evacuated from thespace between the core and the housing to form a vacuum gap 720. Thecombustion chamber is hermetically sealed. Radiative heat transferdominates energy transfer. Exhaust gases do not enter the vacuum gap andare instead routed out from the core, optionally via a jacket that wrapsaround the porous media of the core. Optionally, a porous core 717 andjacket assembly is replaced with a vortex combustor that can alsoachieve high levels of combustion conversion efficiency.

A layer of Aluminum Zinc doped Oxide (AZO) of from about 0.5 mm to about1.6 mm thick optionally coats the external surface 719 of core 717 tosuppress the emissions of far IR spectrum radiation wavelengths, asindicated in FIG. 14, and thus may be useful to reduce the amount ofthermal radiation lost to the surroundings. FIG. 14 provides a plot ofexample spectral radiance form the core of the combustor. The plotfeatures vertical lines indicating where wavelengths of thermal radianceare suppressed or reflected by meta-materials surfaces. Areas of theouter enclosure of portable power generator that do not interface withTWG devices, such as those surrounding core 717 and adjacent to and/orbetween TWGs 200, optionally include surface etching of a meta-materialtexture that reflects radiant energy having wavelengths of from about0.8 μm to about 1.5 μm back to the core. In another embodiment, theradiant energy is reflected towards the locations of TWGs 200 positionedradially from the core 717 of the combustor 715. This is optionallyconfigured using secondary reflectors, photonic crystals, plasmonpolariton resonators, and/or waveguides, for example. Suchconfigurations may advantageously produce highly efficient or nearlyperfect insulation at areas not involved with supplying the thermalpower needed for the operation of TWGs 200.

TWGs 200 are interfaced to the outer housing of the combustor 715.Thermal radiation from the core 717is absorbed using carbon sheets, forexample, of from about 0.5 mm to about 1 mm thick to heat these activeareas, such as to temperatures of from 500° C. to 1100° C. To optimizethe surface coverage of the TWG 200 on its respective heat source, it ispossible to increase the size of a single TWG 200 by increasing the sizeof the individual wafer parts, resulting in an assembled TWG 200 thatis, for example, up to 20 mm in width and up to 300 mm in diameter. Inanother embodiment, it is possible to shrink the design of TWGs 200 toup to 10 mm in width and up to 500 μm in diameter.

Additionally, the electrical power output of the system can be scaled upby electrically connecting individual TWGs 200. Subsequently, the activeareas of combustor 715 can be increased to generate more power from thesystem. The entire system can range in volume from 0.1 L to 1,000 L ofdisplacement or more, for example.

Combustion chemistry and use of a combustor does not limit the utilityof TWGs in conversion of thermal energy to electricity. Other heatsources may be employed, such as concentrated solar, gas turbineengines, rocket engines, and even sub-critical radioactive isotopes, toproduce thermal energy for the TWG to harvest. In embodiments, the TWGmay produce electrical power without the need for refueling until theisotope, for example, decays in a matter of its respective half-life.Long endurance applications found in space, deep sea, and militarymission suit embodiments incorporating radioactive thermal energygeneration.

Illustrations

As used below, any reference to a series of illustrations is to beunderstood as a reference to each of those examples disjunctively.

Illustration 1 is an energy converter comprising: a first electrode foremitting electrons, wherein the first electrode includes a plurality ofmicrotips extending from a base surface of the first electrode, andwherein the first electrode includes an emissive carbon coating over atleast a portion of the microtips; a second electrode adjacent to firstelectrode; a third electrode adjacent to the second electrode, whereinthe second electrode is positioned between the third electrode and thefirst electrode; an inductive element adjacent to the third electrode,wherein the third electrode is positioned between the second electrodeand the inductive element; a fourth electrode for collected electronsemitted from the first electrode, wherein the fourth electrode isadjacent to the inductive element, and wherein the inductive element ispositioned between the third electrode and the fourth electrode; and ahousing defining an enclosed evacuated volume, wherein the firstelectrode, the second electrode, the third electrode, the inductiveelement, and the fourth electrode are positioned within the enclosedevacuated volume.

Illustration 2 is the energy converter of any previous or subsequentillustration, wherein the first electrode comprises a cathode.

Illustration 3 is the energy converter of any previous or subsequentillustration, wherein the first electrode has a lower potential than thefourth electrode.

Illustration 4 is the energy converter of any previous or subsequentillustration, wherein the first electrode comprises a material having awork function of from about 0.25 eV to about 3 eV.

Illustration 5 is the energy converter of any previous or subsequentillustration, wherein the first electrode comprises a crystallineceramic electride or wherein the first electrode comprises a crystallineceramic electride coating.

Illustration 6 is the energy converter of any previous or subsequentillustration, wherein the plurality of microtips exhibit a firstcross-sectional dimension at the base surface of the first electrode anda smaller cross-sectional dimension at a distance from the base surface.

Illustration 7 is the energy converter of any previous or subsequentillustration, wherein the plurality of microtips exhibit a height towidth ratio of from about 4 to about 10.

Illustration 8 is the energy converter of any previous or subsequentillustration, wherein the plurality of microtips exhibit cross-sectionalor height dimensions of from 50 nm to 100 μm.

Illustration 9 is the energy converter of any previous or subsequentillustration, wherein the plurality of microtips comprise verticallyarrayed carbon nanotubes, diamondoids, or alkaline metal oxides.

Illustration 10 is the energy converter of any previous or subsequentillustration, wherein the first electrode is fabricated using one ormore microfabrication techniques.

Illustration 11 is the energy converter of any previous or subsequentillustration, wherein the first electrode is deposited onto a MoCualloysubstrate, and wherein the first electrode has a thickness of from 10 nmto 20 μm.

Illustration 12 is the energy converter of any previous or subsequentillustration, wherein the emissive carbon coating comprises a doped orundoped amorphous carbon coating.

Illustration 13 is the energy converter of any previous or subsequentillustration, wherein the emissive carbon coating comprises a doped orundoped nanodiamond coating.

Illustration 14 is the energy converter of any previous or subsequentillustration, wherein the emissive carbon coating is doped with one ormore of hydrogen, nitrogen, or boron.

Illustration 15 is the energy converter of any previous or subsequentillustration, wherein the emissive carbon coating is deposited using achemical vapor deposition process or a physical vapor depositionprocess.

Illustration 16 is the energy converter of any previous or subsequentillustration, wherein the first electrode has a surface area of from0.05 cm² to 16 cm².

Illustration 17 is the energy converter of any previous or subsequentillustration, wherein the first electrode comprises a refractory metal.

Illustration 18 is the energy converter of any previous or subsequentillustration, wherein the second electrode comprises a control gridelectrode.

Illustration 19 is the energy converter of any previous or subsequentillustration, wherein the second electrode has a potential that variesas a function of time.

Illustration 20 is the energy converter of any previous or subsequentillustration, wherein the second electrode has a lower potential thanthe first electrode.

Illustration 21 is the energy converter of any previous or subsequentillustration, wherein the second electrode is electrically connected tothe first electrode.

Illustration 22 is the energy converter of any previous or subsequentillustration, wherein the second electrode comprises a grid electrode orelectrode array having electrode elements arranged in a gridconfiguration.

Illustration 23 is the energy converter of any previous or subsequentillustration, wherein the second electrode, the third electrode, orboth, covers up to 30% of a surface area of the first electrode, andwherein 70% or more of the surface area of the first electrode isexposed through apertures or spacing regions of the second electrode,the third electrode, or both.

Illustration 24 is the energy converter of any previous or subsequentillustration, wherein the first electrode emits electrons that passthrough apertures or spacing regions of the second electrode.

Illustration 25 is the energy converter of any previous or subsequentillustration, wherein a potential applied to the second electrodemodulates a current of electrons between the first electrode and thefourth electrode.

Illustration 26 is the energy converter of any previous or subsequentillustration, wherein a potential applied to the second electrodefocuses electrons between the first electrode and the fourth electrode.

Illustration 27 is the energy converter of any previous or subsequentillustration, wherein the first electrode, the second electrode, thethird electrode, and/or the fourth electrode is fabricated using one ormore microfabrication techniques.

Illustration 28 is the energy converter of any previous or subsequentillustration, wherein the second electrode is fabricated by patterninglayers of conductive and insulating materials in a grid arrangement.

Illustration 29 is the energy converter of any previous or subsequentillustration, wherein the second electrode is separated from the firstelectrode by an inter electrode spacing of from 5 μm to 400 μm.

Illustration 30 is the energy converter of any previous or subsequentillustration, wherein the third electrode comprises an acceleration gridelectrode.

Illustration 31 is the energy converter of any previous or subsequentillustration, wherein the third electrode has a potential that varies asa function of time.

Illustration 32 is the energy converter of any previous or subsequentillustration, wherein the potential has a square wave modulation with afrequency of from 1 kHz to 1 MHz.

Illustration 33 is the energy converter of any previous or subsequentillustration, wherein the potential has a sinusoidal modulation with afrequency of from about 25 Hz to about 400 Hz.

Illustration 34 is the energy converter of any previous or subsequentillustration, wherein the third electrode has a higher potential thanthe first electrode.

Illustration 35 is the energy converter of any previous or subsequentillustration, wherein the first electrode emits electrons that passthrough apertures of the third electrode.

Illustration 36 is the energy converter of any previous or subsequentillustration, wherein a potential applied to the third electrodemodulates a velocity of electrons passing through apertures of the thirdelectrode.

Illustration 37 is the energy converter of any previous or subsequentillustration, wherein a potential applied to the third electrode focuseselectrons passing through apertures of the third electrode.

Illustration 38 is the energy converter of any previous or subsequentillustration, wherein the third electrode is fabricated using one ormore microfabrication techniques.

Illustration 39 is the energy converter of any previous or subsequentillustration, wherein the inductive element comprises an air coreinductor.

Illustration 40 is the energy converter of any previous or subsequentillustration, wherein the inductive element comprises an air coresolenoid.

Illustration 41 is the energy converter of any previous or subsequentillustration, wherein the inductive element comprises a solid coreinductor.

Illustration 42 is the energy converter of any previous or subsequentillustration, wherein the inductive element comprises a toroidal coreinductor or a pot core inductor.

Illustration 43 is the energy converter of any previous or subsequentillustration, wherein the first electrode emits electrons that passthrough a central opening of the inductive element.

Illustration 44 is the energy converter of any previous or subsequentillustration, wherein electrons passing through an opening of theinductive element induce a current in the inductive element, induce avoltage across the inductive element, and are decelerated byinteractions with the inductive element.

Illustration 45 is the energy converter of any previous or subsequentillustration, wherein the inductive element comprises a conductivematerial arranged in a coil shaped geometry, a planar spiral geometry, azig-zag geometry, or any combination of these.

Illustration 46 is the energy converter of any previous or subsequentillustration, wherein the fourth electrode has a non-planar geometry.

Illustration 47 is the energy converter of any previous or subsequentillustration, wherein the fourth electrode comprises a material having awork function between about 0.25 eV and about 2.5 eV.

Illustration 48 is the energy converter of any previous or subsequentillustration, wherein the fourth electrode comprises a plurality ofstages.

Illustration 49 is the energy converter of any previous or subsequentillustration, wherein the fourth electrode comprises an array ofcollector elements.

Illustration 50 is the energy converter of any previous or subsequentillustration, wherein the collector elements in the array areindividually positioned above openings within the second electrode andthe third electrode.

Illustration 51 is the energy converter of any previous or subsequentillustration, wherein the fourth electrode has a second emissive carboncoating.

Illustration 52 is the energy converter of any previous or subsequentillustration, wherein the second emissive carbon coating comprises adoped or undoped carbon coating.

Illustration 53 is the energy converter of any previous or subsequentillustration, wherein the second emissive carbon coating comprises adoped or undoped nanodiamond coating.

Illustration 54 is the energy converter of any previous or subsequentillustration, wherein the fourth electrode comprises one or morerecessed regions or one or more raised regions.

Illustration 55 is the energy converter of any previous or subsequentillustration, wherein the enclosed evacuated volume has a pressure offrom 10⁻⁵ Torr to 10¹² Torr.

Illustration 56 is the energy converter of any previous or subsequentillustration, further comprising a getter positioned within the enclosedevacuated volume for removing gas from within the enclosed evacuatedvolume.

Illustration 57 is the energy converter of any previous or subsequentillustration, wherein the housing is hermetically sealed.

Illustration 58 is the energy converter of any previous or subsequentillustration, further comprising a magnetic field source positioned todirect a magnetic field between the cathode and the anode.

Illustration 59 is the energy converter of any previous or subsequentillustration, wherein the magnetic field source is a permanent magnet oran electromagnet.

Illustration 60 is the energy converter of any previous or subsequentillustration, further comprising a magnetic core positioned forconcentrating and guiding the magnetic field between the first electrodeand the second electrode.

Illustration 61 is the energy converter of any previous or subsequentillustration, wherein magnetic field lines generated by the magneticfield source are arranged along a direction between the first electrodeand the fourth electrode.

Illustration 62 is the energy converter of any previous or subsequentillustration, wherein magnetic lines generated by the magnetic fieldsource are arranged along a direction parallel to an average electrontrajectory between the first electrode and the fourth electrode.

Illustration 63 is the energy converter of any previous or subsequentillustration, further comprising a thermal conductor in thermalcommunication with the first electrode, the thermal conductor forreceiving radiant energy or thermal energy to heat the first electrode.

Illustration 64 is the energy converter of any previous or subsequentillustration, further comprising one or more electrically insulatingspacer elements between adjacent electrodes.

Illustration 65 is the energy converter of any previous or subsequentillustration, further comprising a switching power supply for generatingvoltages applied to one or more electrodes, wherein the switching powersupply is in electrical communication with the one or more electrodes.

Illustration 66 is the energy converter of any previous or subsequentillustration, further comprising a control circuit in electricalcommunication with one or more electrodes for modulating potentialsapplied to one or more electrodes.

Illustration 67 is the energy converter of any previous or subsequentillustration, further comprising a dynode positioned adjacent to thethird electrode, wherein the dynode is positioned between the thirdelectrode and the inductive element.

Illustration 68 is the energy converter of any previous or subsequentillustration, wherein an inter-element spacing between the dynode andthe third electrode is from about 0.5 μm to about 200 μm.

Illustration 69 is the energy converter of any previous or subsequentillustration, wherein the dynode has a higher potential than the thirdelectrode.

Illustration 70 is the energy converter of any previous or subsequentillustration, wherein the dynode comprises one or more of a metal foil,synthetic diamond, or a secondary electron emission coating.

Illustration 71 is the energy converter of any previous or subsequentillustration, further comprising one or more conductive alloy electrodesfor fixing positions of the first electrode, the second electrode, orthe third electrode.

Illustration 72 is the energy converter of any previous or subsequentillustration, wherein any or all components are fabricated using one ormore microfabrication techniques.

Illustration 73 is a system comprising: a heat source; and a pluralityof energy converters arranged in a spaced configuration about the heatsource.

Illustration 74 is the system of any previous or subsequentillustration, wherein the plurality of energy converters are arrangedradially about the heat source.

Illustration 75 is the system of any previous or subsequentillustration, wherein the heat source comprises a combustor.

Illustration 76 is the system of any previous or subsequentillustration, wherein the plurality of energy converters make up amajority of one or more walls of the combustor.

Illustration 77 is the system of any previous or subsequentillustration, wherein multiple energy converters are arranged in atandem configuration to provide cascading thermal conversion.

Illustration 78 is the system of any previous or subsequentillustration, wherein at least a portion of one or more walls of thecombustor include 3-dimensional surface features that reflectelectromagnetic radiation having wavelengths of from 0.8 μm to 1.5 μm.

Illustration 79 is the system of any previous or subsequentillustration, wherein at least a portion of one or more walls of thecombustor are coated with or comprise aluminum zinc-doped oxide tosuppress emission of black body radiation at wavelengths greater than1.5 μm or from 1.5 μm to 5 μm.

Illustration 80 is the system of any previous or subsequentillustration, wherein the combustor has a temperature of from about 400°C. to about 1100° C. and generates black body electromagnetic radiation,and wherein at least a portion of the black body electromagneticradiation is transmitted and absorbed by the plurality of energyconverters to heat portions of the plurality of energy converters.

Illustration 81 is the system of any previous illustration, wherein oneor more of the plurality of energy converters independently comprise anenergy converter of any previous illustration.

REFERENCES

U.S. Pat. Nos. 3,265,910, 3,328,611, 3,519,854, 3,702,408, 4,303,845,4,323,808, 5,459,367, 5,780,954, 5,810,980, 5,942,834, 5,981,071,5,994,638, 6,103,298, 6,211,465, 6,229,083, 6,495,843, 8,853,531,9,865,789, 8,853,531, and 3,460,524 are hereby incorporated byreference.

Lauren Rand, John Williams, Joseph Blakely, Brian Beal, and DanielBrown, “C12A7 Electride Hollow Cathode,” Conference paper for the JANNAFSpace Subcommittee meeting, Colorado Springs, Colo., 29 Apr.-3 May 2013is hereby incorporated by reference.

Statements Regarding Incorporation by Reference and Variations

All references throughout this application, for example patent documentsincluding issued or granted patents or equivalents; patent applicationpublications; and non-patent literature documents or other sourcematerial; are hereby incorporated by reference herein in theirentireties, as though individually incorporated by reference.

All patents and publications mentioned in the specification areindicative of the levels of skill of those skilled in the art to whichthe invention pertains. References cited herein are incorporated byreference herein in their entirety to indicate the state of the art, insome cases as of their filing date, and it is intended that thisinformation can be employed herein, if needed, to exclude (for example,to disclaim) specific embodiments that are in the prior art. Forexample, when a compound is claimed, it should be understood thatcompounds known in the prior art, including certain compounds disclosedin the references disclosed herein (particularly in referenced patentdocuments), are not intended to be included in the claim.

When a group of substituents is disclosed herein, it is understood thatall individual members of those groups and all subgroups and classesthat can be formed using the substituents are disclosed separately. Whena Markush group or other grouping is used herein, all individual membersof the group and all combinations and subcombinations possible of thegroup are intended to be individually included in the disclosure. Asused herein, “and/or” means that one, all, or any combination of itemsin a list separated by “and/or” are included in the list; for example“1, 2 and/or 3” is equivalent to “‘1’ or ‘2’ or ‘3’ or ‘1 and 2’ or ‘1and 3’ or ‘2 and 3’ or ‘1, 2 and 3’”.

Every formulation or combination of components described or exemplifiedcan be used to practice the invention, unless otherwise stated. Specificnames of materials are intended to be exemplary, as it is known that oneof ordinary skill in the art can name the same material differently. Oneof ordinary skill in the art will appreciate that methods, deviceelements, starting materials, and techniques other than thosespecifically exemplified can be employed in the practice of theinvention without resort to undue experimentation. All art-knownfunctional equivalents, of any such methods, device elements, startingmaterials, and techniques are intended to be included in this invention.Whenever a range is given in the specification, for example, atemperature range, a time range, or a composition range, allintermediate ranges and subranges, as well as all individual valuesincluded in the ranges given are intended to be included in thedisclosure.

As used herein, “comprising” is synonymous with “including,”“containing,” or “characterized by,” and is inclusive or open-ended anddoes not exclude additional, unrecited elements or method steps. As usedherein, “consisting of” excludes any element, step, or ingredient notspecified in the claim element. As used herein, “consisting essentiallyof” does not exclude materials or steps that do not materially affectthe basic and novel characteristics of the claim. Any recitation hereinof the term “comprising”, particularly in a description of components ofa composition or in a description of elements of a device, is understoodto encompass those compositions and methods consisting essentially ofand consisting of the recited components or elements. The inventionillustratively described herein suitably may be practiced in the absenceof any element or elements, limitation or limitations which is notspecifically disclosed herein.

The terms and expressions which have been employed are used as terms ofdescription and illustration and not of limitation, and there is nointention in the use of such terms and expressions of excluding anyequivalents of the features shown and described or portions thereof. Itwill be recognized that various modifications are possible within thescope of the invention claimed. Thus, it should be understood thatalthough the present invention has been specifically disclosed bypreferred embodiments and optional features, modification and variationof the concepts herein disclosed may be resorted to by those skilled inthe art, and that such modifications and variations are considered to bewithin the scope of this invention as defined by the appended claims.

What is claimed is:
 1. An energy converter comprising: a cathode foremitting electrons; an anode for collected electrons emitted from thecathode; an inductive element positioned to electromagnetically interactwith electrons emitted by the cathode; a magnetic field sourcepositioned to direct a magnetic field between the cathode and the anode;and a housing defining an enclosed volume, wherein the cathode, theinductive element, and the anode are positioned within the enclosedvolume.
 2. The energy converter of claim 1, further comprising a controlgrid electrode between the cathode and the anode.
 3. The energyconverter of claim 1, further comprising an acceleration grid electrodebetween the cathode and the anode.
 4. The energy converter of claim 1,wherein the cathode comprises a crystalline ceramic electride or acrystalline ceramic electride coating.
 5. The energy converter of claim1, wherein the cathode includes a plurality of microtips extending froma base surface of the cathode, or wherein the cathode comprises anemissive carbon coating, a doped amorphous carbon coating, or an undopedamorphous carbon coating.
 6. The energy converter of claim 1, furthercomprising a control circuit in electrical communication with one ormore of the cathode, the anode, or the inductive element for modulatingone or more potentials applied thereto.
 7. The energy converter of claim6, wherein the control circuit is configured to apply a potential thatmodulates a current of electrons between the cathode and the anode orfocuses electrons between the cathode and the anode.
 8. The energyconverter of claim 6, wherein the control circuit is configured to applya potential that varies as a function of time for modulating a velocityof electrons travelling from the cathode to the anode.
 9. The energyconverter of claim 6, wherein the control circuit is configured tocontrol the one or more potentials to generate an AC voltage or currentoutput from the anode having a frequency of 50 Hz or 60 Hz.
 10. Theenergy converter of claim 6, wherein the control circuit is configuredto control the one or more potentials to generate an DC voltage orcurrent output from the anode.
 11. The energy converter of claim 1,wherein the magnetic field source comprises a permanent magnet or anelectromagnet.
 12. The energy converter of claim 1, further comprising amagnetic core positioned for concentrating and guiding the magneticfield between the cathode and the anode.
 13. The energy converter ofclaim 1, further comprising a thermal conductor in thermal communicationwith the cathode, the thermal conductor positioned to receive radiantenergy or thermal energy and to heat the cathode.
 14. The energyconverter of claim 1, wherein the enclosed volume is evacuated to apressure of from 10⁻⁵ Torr to 10⁻¹² Torr.
 15. The energy converter ofclaim 1, further comprising a getter positioned within the enclosedvolume.
 16. A system comprising: a heat source; and a plurality ofenergy converters arranged in a spaced configuration about the heatsource, wherein one or more of the plurality of energy convertersindependently comprise the energy converter of claim
 1. 17. The systemof claim 16, wherein the heat source comprises a combustor.
 18. Thesystem of claim 17, wherein at least a portion of one or more walls ofthe combustor include 3-dimensional surface features that reflectelectromagnetic radiation having wavelengths of from 0.8 μm to 1.5 μm.19. The system of claim 17, wherein at least a portion of one or morewalls of the combustor are coated with or comprise aluminum zinc-dopedoxide to suppress emission of black body radiation at wavelengthsgreater than 1.5 μm or from 1.5 μm to 5 μm.
 20. The system of claim 16,wherein the heat source has a temperature of from about 400° C. to about1100° C. and generates black body electromagnetic radiation, and whereinat least a portion of the black body electromagnetic radiation istransmitted and absorbed by the plurality of energy converters to heatcathodes of the plurality of energy converters.